It is well known that for the color development of a silver halide photographic light-sensitive material, after exposure, an oxidized aromatic primary amine developing agent can be reacted with a dye forming coupler to obtain a color image.
It is also known that, for the color development of a silver halide color photographic material, an oxidized aromatic primary amine color developing agent can be reacted with a coupler to form a dye such as an indophenol, an indoaniline, an indamine, an azomethine, a phenoxazine, a phenazine, and the like, thus forming a color image. In this procedure, the subtractive color process is ordinarily used for color reproduction, and silver halide emulsions which are selectively sensitive to blue, green, and red light, and yellow, magenta, and cyan color image formers, which are respectively the complementary colors of blue, green, and red, are employed. For example, a coupler of the acylacetanilide or benzoylmethane type is used for forming a yellow color image; a coupler of the pyrazolone, pyrazolobenzimidazole, cyanoacetophenone or indazolone type is generally used for forming a magenta color image; and a phenolic coupler, such as a phenol and a naphthol, is generally used for forming a cyan color image.
It is also known that, in addition to color couplers which do not have a substituent at the coupling position thereof, and thus require development of 4 mols of exposed silver halide for forming 1 mol of a dye, there are color couplers which have a substituent capable of being released upon color development at the coupling position thereof, and thus require development of only 2 mols of exposed silver halide for forming 1 mol of a dye. The former are known as 4-equivalent color couplers, and the latter are known as 2-equivalent color couplers.
Color couplers must satisfy various requirements. For example, it is necessary that they provide a dye image having a good spectral property and excellent stability to light, temperature, and humidity for a long period of time upon color development.
It is also required in a multilayer color photographic light-sensitive material that each coupler is fixed in a layer separated from each other in order to reduce color mixing and to improve color reproduction. Many methods for rendering a coupler diffusion-resistant are known. One method is to introduce a long chain aliphatic group into a coupler molecule in order to prevent diffusion. Couplers according to such a method require a step of addition to an aqueous gelatin solution by solubilizing in alkali, or a step of dispersing in an aqueous gelatin solution by dissolving in a high boiling point organic solvent, since the couplers are immiscible with an aqueous gelatin solution. Such color couplers may cause crystal formation in a photographic emulsion and, in the case of using a high boiling point organic solvent, a large amount of gelatin must be employed since the high boiling point organic solvent makes an emulsion layer soft. Consequently, this brings about an opposite result to the requirement in that a thickness of the emulsion layer be reduced.
Another method for rendering a coupler diffusion-resistant is to utilize a polymer coupler obtained by polymerization of a monomeric coupler. However, such polymer couplers have disadvantages, in that they have poor solubility to water, and they increase the viscosity of an aqueous gelatin solution. In order to overcome these disadvantages, polymer coupler latexes have been provided. The method of adding a polymer coupler in a latex form to a hydrophilic colloid composition has many advantages in comparison with other methods. For example, when the polymer coupler latex is used, the step of adding the coupler to a coating solution can be simplified, since the use of a high boiling point organic solvent or an alkali is not necessary and a special dispersing method is not required. Further, the deterioration of strength of the film formed is small, because the hydrophobic substance is in a latex form. Also, the thickness of the layer can be reduced, since an organic solvent is not contained therein. Furthermore, since the latex can contain coupler monomers in a high concentration, it is easy to incorporate couplers in a high concentration into a photographic emulsion, and the increase of viscosity is small. Moreover, color mixing is prevented, since a polymer coupler is completely immobilized. Accordingly, there is only a small amount of coupler crystallization in the emulsion layer.
Examples of polymer couplers latexes described above include a 4-equivalent magenta polymer coupler latex (methods of preparation are disclosed in U.S. Pat. Nos. 3,451,820 and 4,080,211 and British Pat. No. 1,247,688), a copolymer latex of a competing coupler (as disclosed in West German Pat. No. 2,725,591 and U.S. Pat. No. 3,926,436), and a cyan polymer coupler latex (as disclosed in U.S. Pat. No. 3,767,412).
However, these polymer coupler latexes have a number of problems in addition to the many advantages described above. The problems include the following:
With respect to the polymerization technique, it should be noted that since a solid monomer coupler has low solubility, crystallization of the monomer coupler and coagulation of a latex occur, which prevents polymerization in a high concentration.
Emulsion polymerization methods of a solid water-insoluble monomer coupler in water are described in U.S. Pat. No. 4,080,211 and Belgian Pat. No. 669,971.
A first polymerization method comprises dissolving a solid water-insoluble monomer coupler in an ethylenically unsaturated copolymerizable monomer and a water-miscible or water-immiscible organic solvent, then adding the resulting solution to an aqueous reaction medium containing an emulsifier and initiating polymerization. The organic solvent capable of using in this method, however, must satisfy the following requirements: (1) it is substantially inert to the solid water-insoluble monomer coupler, (2) it does not interrupt the normal action of the free-radical addition polymerization, and (3) it has a low boiling point which makes it possible to easily remove it from the aqueous reaction medium by distillation during and/or after the polymerization.
A second polymerization method comprises dissolving a solid water-insoluble monomer coupler in an ethylenically unsaturated copolymerizable monomer, then adding the resulting solution to an aqueous reaction medium containing an emulsifier and initiating polymerization.
A third polymerization method comprises dispersing a solid water-insoluble monomer coupler and an ethylenically unsaturated copolymerizable monomer or a solid water-insoluble monomer coupler, an ethylenically unsaturated copolymerizable monomer and an organic solvent in an aqueous reaction medium containing an emulsifier and initiating polymerization.
However, each of these methods have disadvantages. More specifically, the first method is not desirable because large amount of the organic solvent is needed to dissolve the solid water-insoluble monomer coupler and a large amount of energy and time is required in order to remove the organic solvent employed. The second method is not desirable became the solid water-insoluble monomer coupler is apt to crystallize since the ethylenically unsaturated copolymerizable monomer has only limited solubility. The third method is not desirable because the solid water-insoluble monomer coupler is dispersed in a solid form and it is hardly dissolved in the aqueous reaction medium. Accordingly, a large amount of aggregates remain undispersed.
Further, the polymer coupler latexes prepared by polymerization by the first, second or third method have the following disadvantages.
1. The rate of the coupling reaction is poor, and thus the density of dye formed is very low.
2. Undesirable fog is readily formed upon color development.
3. The latexes cannot be stored for a long period of time since aggregation occurs in solution